Hades Breaching Drill
Part of the Kriegers' siege doctrine is to dig a tunnel under enemy lines; when they're almost up to the surface, they bring in the Hades to chew through the last few yards (and anything unlucky enough to be wherever the Hades emerges). While the enemy is busy dealing with the whirling melta-cutter, a team of Engineers can slip in through the tunnel and cause even more trouble. This all has the effect of distracting the enemy from its external defenses, so the Kriegers waiting in the trenches outside will take advantage of the enemy's confusion and press the assault.
For all their melta-cutting goodness, it turns out that Hades Breaching Drills have light armor; the Imperial Guard has tried to use it to dig through above-ground fortifications, but this has never really worked well.
One might wonder why the Hades isn't used to drill the entire tunnel; it is possible that the Kriegers think that it will be detected, but it may just be that there are many more Kriegers than Hades Breaching Drills, and that tunnels are more rapidly and cheaply dug the old-fashioned way. There are several reasons they don't though, when the drills are in action they are loud and create a lot of vibration and - somewhat importantly - tunnels go both ways. With even a few minutes warning the enemy can not just ambush your attacking dudes, they can push you back all the way to your HQ which is something of a tactical downside. So they dig the tunnel to as close as they can without being heard by hand then bring up the drill to smash through with no warning at all.
So, the Hades got a huge change in the second edition of IA5. Instead of being a Rhino-grade armor, 2HP vehicle with a melta-blast gun, it's an Infantry model with a Monstrous Creature grade statline (S8, T7, 3 Wounds, 3+ armor), that is bought as a sort of upgrade character slash transport for the Siege Engineers. Unlike other transports, it's actually attached to the squad and has to be in coherency with it at all times. It still has to arrive from Reserves, this time using the Deep Strike rules. But on the turn it does, it gets a 4+ invulnerable save. It also has the Move Through Cover and Hammer of Wrath rules by default - a charging Hades can dump a S8 hit at I10 on whatever it gets into base contact with, and that's going to hurt a lot. But the Hades also got some nifty bonuses too. It gets a new rule called "Tunneling", usable in either the Movement or Assault phases. In Movement, you can move 12" in any direction, ignoring terrain and models, and gains a 4+ invulnerable save until your next Movement phase rolls around. The catch is that you can't run, charge, or be in impassable terrain or within 2" of an enemy model when you end the move. But 12" movement that ignores any terrain or models that are in the way? That's pretty bitchin'. Alternately, you can use Tunneling in the Assault phase. You have to use 3D6 and pick the two lowest results for your charge move, but the unit counts as having assault grenades, ignores terrain and battlefield debris effects, and Overwatch can't be fired against them. Oh, and the Hades? It gets to make D6+2 HoW attacks, using its Meltacutter drill, a S8/AP1 Armorbane & Shred close combat weapon. Plus, if you charge a building, anybody inside is going to take a S4/AP2 hit on a 4+. Are you seeing those Devastators camping in that bunker? They are calling for a can opener! Alas, the Hades does have a fatal flaw. If all the Engineers in the unit are killed, then the Hades is removed regardless of it having any Wounds remaining. Flipside is, that you can kill off the majority of the Engineers and then end up with majority-Toughness 7. Being able to invalidate Overwatch is also badass, and a great "fuck you" to shitfaced Tau players and their Supporting Fire.